Waste heat is often created as a byproduct of industrial processes where flowing streams of high-temperature liquids, gases, or fluids must be exhausted into the environment or removed in some way in an effort to maintain the operating temperatures of the industrial process equipment. Some industrial processes utilize heat exchanger devices to capture and recycle waste heat back into the process via other process streams. However, the capturing and recycling of waste heat is generally infeasible by industrial processes that utilize high temperatures or have insufficient mass flow or other unfavorable conditions.
Waste heat can be converted into useful energy by a variety of turbine generator or heat engine systems that employ thermodynamic methods, such as Rankine cycles. Rankine cycles and similar thermodynamic methods are typically steam-based processes that recover and utilize waste heat to generate steam for driving a turbine, turbo, or other expander connected to an electric generator, a pump, or other device.
An organic Rankine cycle utilizes a lower boiling-point working fluid, instead of water, during a traditional Rankine cycle. Exemplary lower boiling-point working fluids include hydrocarbons, such as light hydrocarbons (e.g., propane or butane) and halogenated hydrocarbon, such as hydrochlorofluorocarbons (HCFCs) or hydrofluorocarbons (HFCs) (e.g., R245fa). More recently, in view of issues such as thermal instability, toxicity, flammability, and production cost of the lower boiling-point working fluids, some thermodynamic cycles have been modified to circulate non-hydrocarbon working fluids, such as ammonia.
A pump or compressor is generally required to pressurize and circulate the working fluid throughout the working fluid circuit. The pump is typically a motor-driven pump, however, such pumps require costly shaft seals to prevent working fluid leakage and often require the implementation of a gearbox and a variable frequency drive, which add to the overall cost and complexity of the system. A turbo pump is a device that utilizes a drive turbine to power a rotodynamic pump. Replacing the motor-driven pump with a turbo pump eliminates one or more of these issues, but at the same time introduces problems of starting and achieving steady-state operation the turbo pump, which relies on the circulation of heated working fluid through the drive turbine for proper operation. Unless the turbo pump is provided with a successful start sequence, the turbo pump will not be able to circulate enough fluid to properly function and attain steady-state operation.
What is needed, therefore, is a heat engine system and method of operating a waste heat recovery thermodynamic cycle that provides a successful start sequence adapted to start a turbo pump and reach a steady-state of operating the system with the turbo pump.